Method for producing recycled acrylic resin and acrylic resin film

Abstract

To provide a method for producing a recycled acrylic resin which makes a removal ratio of an easily adhesive layer high, and can suppress hydrolysis.SOLUTION: A method for producing a recycled acrylic resin removes an easily adhesive layer from an acrylic resin base material to which an easily adhesive layer is attached, and produces a recycled acrylic resin. The method for producing the recycled acrylic resin includes a step of immersing the acrylic resin base material to which the easily adhesive layer is attached in alkali aqueous solution, and subjecting the acrylic resin base material to alkali treatment. The acrylic resin contained in the acrylic resin base material has syndiotacticity expressed by triad syndiotacticity of 54% or more. In the method for producing the recycled acrylic resin, the temperature of the alkali aqueous solution is 30°C or higher and 90°C or lower, and a processing parameter expressed by a product of the temperature of the alkali aqueous solution and a time required for immersion in the alkali aqueous solution is 2,000°C min or more and 10,000°C min or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing recycled acrylic resin and an acrylic resin film. [Background technology]

[0002] In a liquid crystal display device, two polarizers are usually arranged on both sides of a liquid crystal cell, and the polarizers are provided with polarizer protective films to protect their surfaces. An acrylic film is sometimes used as the polarizer protective film (see Patent Documents 1 and 2). In addition, an easy-adhesion layer may be formed on one side of the acrylic film to improve adhesion between the acrylic film and the polarizer (see Patent Document 3). In this case, the polarizer protective film is adhered to the polarizer by applying an adhesive to the easy-adhesion layer.

[0003] In recent years, from the perspective of effective utilization of materials, efforts have been made to recycle resins by using film scraps as part of the raw materials. However, when using scraps as part of the raw materials to produce resin for optical films using acrylic films with easy-adhesion layers, it is necessary to remove the easy-adhesion layer to prevent defects in the optical film that may occur due to the easy-adhesion layer remaining as foreign matter.

[0004] Here, as a method for removing a layer (heterogeneous layer) made of a resin different from the substrate and laminated on the substrate made of resin, for example, Patent Document 4 discloses a method in which a heterogeneous layer provided on a substrate made of a synthetic resin or a cellulose organic acid ester is removed in an alkaline aqueous solution at 105°C or higher, and then the alkaline aqueous solution is removed from the substrate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2023 / 238885 [Patent Document 2] International Publication No. 2023 / 238886 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-55062 [Patent Document 4] Japanese Patent Application Laid-Open No. 2001-310970 Summary of the Invention [Problem to be solved by the invention]

[0006] The method described in Patent Document 4 can also be applied to the recycling of acrylic films having an easy-adhesion layer. However, when the acrylic resin constituting the acrylic film has a high syndiotacticity in triad expression, increasing the temperature of the alkaline aqueous solution or extending the treatment time increases the removal rate of the easy-adhesion layer, but may increase the hydrolysis rate of the recycled acrylic resin.

[0007] An object of the present invention is to provide a method for producing recycled acrylic resin that can increase the removal rate of the easy-adhesion layer and suppress hydrolysis. [Means for solving the problem]

[0008] (1) A method for producing recycled acrylic resin by removing an easy-adhesion layer from an acrylic resin substrate to which the easy-adhesion layer is attached, the method comprising the step of immersing the acrylic resin substrate to which the easy-adhesion layer is attached in an alkaline aqueous solution to perform an alkaline treatment, wherein the acrylic resin contained in the acrylic resin substrate has a syndiotacticity of 54% or more, expressed as a triplet, and the temperature of the alkaline aqueous solution is 30°C or more and 90°C or less, and the treatment parameter, which is expressed as the product of the temperature of the alkaline aqueous solution and the time for immersion in the alkaline aqueous solution, is 2000°C·min or more and 10000°C·min or less.

[0009] (2) The method for producing a recycled acrylic resin according to (1), wherein the acrylic resin has a syndiotacticity of 55% or more in triad expression.

[0010] (3) The method for producing a recycled acrylic resin according to (1) or (2), wherein the acrylic resin has a content of structural units derived from methyl methacrylate of 98% by weight or more.

[0011] (4) The method for producing a recycled acrylic resin according to any one of (1) to (3), wherein the acrylic resin does not contain a ring structure in the main chain.

[0012] (5) The method for producing a recycled acrylic resin according to any one of (1) to (4), wherein the easy-adhesion layer contains a urethane resin.

[0013] (6) An acrylic resin film having a glass transition temperature of 120°C or higher and a contact angle with water at 25°C of 65.0° or higher and 71.0° or lower.

[0014] (7) An acrylic resin film having a triad syndiotacticity of 54% or more and a contact angle with water at 25°C of 65.0° or more and 71.0° or less. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a method for producing recycled acrylic resin that can increase the removal rate of the easy-adhesion layer and suppress hydrolysis. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of the present invention will be described.

[0017] [Manufacturing method for recycled acrylic resin] The method for producing recycled acrylic resin of this embodiment involves removing an easy-adhesion layer from an acrylic resin substrate to which the easy-adhesion layer is attached to produce recycled acrylic resin. Specifically, the method for producing recycled acrylic resin of this embodiment includes a step of immersing the acrylic resin substrate to which the easy-adhesion layer is attached in an alkaline aqueous solution to perform an alkali treatment. Here, the acrylic resin contained in the acrylic resin substrate has a syndiotacticity of 54% or more, expressed as a triad. The temperature of the alkaline aqueous solution is 30°C or higher and 90°C or lower. If the temperature of the alkaline aqueous solution is lower than 30°C, the removal rate of the easy-adhesion layer decreases, and if it exceeds 90°C, the hydrolysis rate of the recycled acrylic resin increases. Furthermore, the treatment parameter, which is the product of the temperature of the alkaline aqueous solution and the immersion time in the alkaline aqueous solution, is 2000°C·min or higher and 10,000°C·min or lower. If the treatment parameter is lower than 2000°C·min, the removal rate of the easy-adhesion layer decreases, and if it exceeds 10,000°C·min, the hydrolysis rate of the recycled acrylic resin increases. If the hydrolysis rate of the recycled acrylic resin becomes too high, the thermal decomposition resistance of the recycled acrylic resin may decrease, and the recycled acrylic resin may react to form gels or other imperfections when the recycled acrylic resin film is remolded, which is undesirable. The hydrolysis rate of the recycled acrylic resin is evaluated by measuring the contact angle of the recycled acrylic resin film with water.

[0018] In this specification and claims, acrylic resin refers to a polymer of a monomer having an acryloyl group and / or a monomer having a methacryloyl group. In this case, the acrylic resin may be either a homopolymer or a copolymer. When the acrylic resin is a copolymer, it may also be a copolymer of a monomer not having an acryloyl group or a methacryloyl group.

[0019] The temperature of the alkaline aqueous solution is from 30°C to 90°C, preferably from 45°C to 85°C, and more preferably from 60°C to 80°C. The processing parameters are from 2000°C·min to 10000°C·min, preferably from 3300°C·min to 8000°C·min, and more preferably from 3600°C·min to 7200°C·min.

[0020] The time for immersion in the alkaline aqueous solution is not particularly limited, but is, for example, from 60 minutes to 180 minutes.

[0021] The base contained in the alkaline aqueous solution is not particularly limited, but examples thereof include sodium hydroxide, lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, etc., and two or more of these may be used in combination. Among these, lithium hydroxide, sodium hydroxide, and potassium hydroxide are preferred, and sodium hydroxide and potassium hydroxide are particularly preferred.

[0022] The pH of the alkaline aqueous solution is not particularly limited, but is, for example, 10 or more and 14 or less.

[0023] The alkaline aqueous solution may or may not contain a surfactant.

[0024] In the present embodiment, the acrylic resin substrate having the easy-adhesion layer attached thereto can be, for example, crushed scraps generated during the production of a polarizer protective film having an easy-adhesion layer formed on one side of an acrylic film. In this case, the scraps may be crushed and then sized.

[0025] The thickness of the acrylic film is not particularly limited, but is, for example, 10 μm to 100 μm, and the thickness of the easy-adhesion layer is not particularly limited, but is, for example, 0.2 μm to 0.4 μm.

[0026] The acrylic film can be obtained, for example, by stretching a raw film having an easy-adhesion layer formed on one side thereof, or by stretching the raw film and then forming an easy-adhesion layer on one side thereof.

[0027] The method for producing recycled acrylic resin of this embodiment may further include a step of filtering the alkaline aqueous solution in which the acrylic resin substrate having the easy-adhesion layer attached thereto has been immersed, a step of washing the filtered residue with water, and a step of drying the washed filtered residue. In this step, the acrylic resin substrate having the easy-adhesion layer attached thereto may be sized while being immersed in the alkaline aqueous solution, or the dried filtered residue may be sized.

[0028] The recycled acrylic resin produced by the method for producing recycled acrylic resin of the present embodiment is mixed with virgin acrylic resin as needed before use.

[0029] Recycled acrylic resins can be applied to, but are not particularly limited to, electronic materials, etc. For example, films containing recycled acrylic resins can be used as antenna substrates, display substrates, touch panel substrates, etc. Examples of display substrates include liquid crystal display films, surface protective films, polarizer protective films, etc. Films containing recycled acrylic resins can be particularly suitably used in known optical applications such as in the periphery of liquid crystal display devices, such as optically isotropic films, polarizer protective films, and transparent conductive films, taking advantage of their optical properties. Furthermore, films containing recycled acrylic resins can be attached to polarizers to be used as polarizing plates. That is, films containing recycled acrylic resins can be used as polarizer protective films for polarizing plates.

[0030] Furthermore, the recycled acrylic resin produced by the recycled acrylic resin production method of this embodiment is expected to be suitable for chemical recycling (e.g., a method in which cracked oil is recovered as a cracked product by thermal decomposition and reused as a chemical raw material or fuel). Generally, to improve the heat resistance and thermal stability of acrylic resins, a ring structure is introduced into the main chain of the acrylic resin or a monomer having a rigid structure is copolymerized. However, these structures become impurities when chemically recycling the acrylic resin, and are therefore undesirable. In this regard, the recycled acrylic resin produced by the recycled acrylic resin production method of this embodiment has a high content of structural units derived from methyl methacrylate, and therefore is expected to have a high yield of monomers recovered as cracked oil, and therefore exhibit good chemical recyclability.

[0031] [Acrylic resin] The syndiotacticity, expressed as a triad, of the acrylic resin contained in the acrylic resin substrate is 54% or more, preferably 55% or more, more preferably 56% or more, and even more preferably 57% or more. When the syndiotacticity, expressed as a triad, of the acrylic resin contained in the acrylic resin substrate is 54% or more, the glass transition temperature of the acrylic resin tends to be high and the heat resistance tends to be improved. Furthermore, the syndiotacticity, expressed as a triad, of the acrylic resin contained in the acrylic resin substrate is preferably 80% or less, and from the viewpoints of the molding temperature of the recycled acrylic resin, the toughness of the molded body, and secondary processability, is more preferably 67% or less, even more preferably 65% or less, and even more preferably 63% or less.

[0032] The syndiotacticity of acrylic resin triads is the proportion of three structural unit sequences (triads) that are rr. In addition, in two structural unit sequences (diads), those with the same configuration are called meso (m) and those with the opposite configuration are called racemo (r).

[0033] The glass transition temperature of the acrylic resin is preferably 120° C. or higher, more preferably over 120° C., even more preferably 121° C. or higher, and particularly preferably 122° C. or higher. From the viewpoints of the molding temperature of the recycled acrylic resin and the secondary processability of the molded body, the glass transition temperature of the acrylic resin is preferably 135° C. or lower, more preferably 130° C. or lower.

[0034] The content of structural units derived from methyl methacrylate in the acrylic resin is preferably 98% by weight or more, more preferably 99% by weight or more, and even more preferably 100% by weight.

[0035] The monomer other than methyl methacrylate that constitutes the acrylic resin is not particularly limited, but examples thereof include alkyl acrylate esters such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate; aryl acrylate esters such as phenyl acrylate; cycloalkyl acrylate esters such as cyclohexyl acrylate and norbornenyl acrylate; alkyl methacrylate esters other than methyl methacrylate such as ethyl methacrylate, propyl methacrylate, and butyl methacrylate; aryl methacrylate esters such as phenyl methacrylate; cycloalkyl methacrylate esters such as cyclohexyl methacrylate and norbornenyl methacrylate; aromatic vinyl compounds such as styrene and α-methylstyrene; acrylamide; methacrylamide; acrylonitrile; and methacrylonitrile.

[0036] The weight-average molecular weight of the acrylic resin is preferably from 50,000 to 200,000, more preferably from 70,000 to 180,000, even more preferably from 80,000 to 160,000, and even more preferably from 90,000 to 150,000. When the weight-average molecular weight of the acrylic resin is 50,000 or more, the mechanical properties of molded articles of the recycled acrylic resin tend to improve, and when it is 200,000 or less, the moldability of the recycled acrylic resin tends to improve.

[0037] The weight-average molecular weight of the acrylic resin may be 400,000 or more. When the weight-average molecular weight of the acrylic resin is 400,000 or more, the mechanical properties of the molded product of the recycled acrylic resin tend to be further improved, and for example, a resin film with excellent flex resistance can be obtained. In this case, the weight-average molecular weight of the acrylic resin is preferably 600,000 or more, more preferably 700,000 or more, and even more preferably 800,000 or more. There is no particular upper limit for the weight-average molecular weight (Mw), but from the viewpoint of moldability, it is preferably 4,000,000 or less, more preferably 3,500,000 or less, even more preferably 3,000,000 or less, particularly preferably 2,000,000 or less, and extremely preferably 1,500,000 or less.

[0038] The ratio of the weight average molecular weight to the number average molecular weight of the acrylic resin (dispersity) is preferably 1.6 to 2.5, more preferably 1.7 to 2.2. When the dispersity of the acrylic resin is 1.6 or more, the fluidity of the recycled acrylic resin tends to improve and it tends to be easier to mold, while when it is 2.5 or less, the mechanical properties of molded articles of the recycled acrylic resin, such as impact resistance, toughness, and flex resistance, tend to improve.

[0039] The number-average molecular weight and weight-average molecular weight of the acrylic resin are values measured by gel permeation chromatography (GPC) in terms of standard polystyrene. The number-average molecular weight and weight-average molecular weight of the acrylic resin can be controlled by the type and amount of polymerization initiator and chain transfer agent used in synthesizing the acrylic resin.

[0040] The synthesis method of the acrylic resin is not particularly limited, but examples thereof include anionic polymerization and radical polymerization. Among these, radical polymerization is preferred (see, for example, Patent Documents 1 and 2). Anionic polymerization uses an organometallic compound as a polymerization initiator and an organic solvent as a medium, which is undesirable from the standpoint of leaving impurities behind and being environmentally friendly. The syndiotacticity of the acrylic resin can be controlled by the polymerization temperature of the acrylic resin. For example, lowering the polymerization temperature of the acrylic resin increases the syndiotacticity of the acrylic resin.

[0041] [Easy adhesive layer] The easy-adhesion layer contains a resin and may further contain a crosslinking agent. The easy-adhesion layer is formed, for example, by applying an easy-adhesion adhesive containing a resin and then drying it. Here, the resin may be a thermosetting resin or a thermoplastic resin. Furthermore, the easy-adhesion layer may or may not have thermosetting properties by itself.

[0042] The resin is not particularly limited, but examples thereof include urethane resin, epoxy resin, etc. Among these, urethane resin is preferred.

[0043] The urethane resin is not particularly limited, and known materials can be used (see, for example, Patent Document 2). The urethane resin preferably has a carboxyl group. Examples of urethane resins having a carboxyl group include polyester urethane resins having a carboxyl group. Examples of commercially available polyester urethane resins having a carboxyl group include Superflex 210 (manufactured by Dai-ichi Kogyo Seiyaku).

[0044] The crosslinking agent is not particularly limited as long as it is a material capable of crosslinking with the resin, and known materials can be used (see, for example, Patent Document 2). Examples of materials capable of crosslinking with a urethane resin having a carboxyl group include compounds and polymers having an epoxy group, a carbodiimide group, or an oxazoline group. Examples of commercially available polymers having an oxazoline group include Epocross WS700 (manufactured by Nippon Shokubai).

[0045] [Acrylic resin film] The contact angle of the acrylic resin film of this embodiment with water at 25° C. is 65.0° or more and 71.5° or less, preferably 65.4° or more and 70.8° or less, and more preferably 67.0° or more and 70.0° or less. The acrylic resin film of this embodiment is molded by a known molding method using, for example, recycled acrylic resin produced by the method for producing recycled acrylic resin of this embodiment.

[0046] The acrylic resin film of this embodiment may have a glass transition temperature of 120° C. or higher. In this case, the glass transition temperature of the acrylic resin film of this embodiment is preferably higher than 120° C., more preferably 121° C. or higher, and even more preferably 122° C. or higher. From the viewpoint of secondary processability, the glass transition temperature of the acrylic resin film of this embodiment is preferably 135° C. or lower, and more preferably 130° C. or lower.

[0047] The acrylic resin film of this embodiment may have a syndiotacticity of 54% or more, expressed as a triad. In this case, the syndiotacticity of the acrylic resin film of this embodiment is preferably 55% or more, more preferably 56% or more, and even more preferably 57% or more. The acrylic resin film of this embodiment preferably contains an acrylic resin having a content of structural units derived from methyl methacrylate of 98% by weight or more. Furthermore, the acrylic resin film of this embodiment preferably has a glass transition temperature of 120°C or more.

[0048] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and the above-described embodiments may be modified as appropriate within the scope of the spirit of the present invention. [Example]

[0049] The present invention will be described in more detail below with reference to examples and comparative examples. However, the present invention is not limited to these examples. The methods for measuring various physical properties are as follows.

[0050] (conversion rate) The weight ratio of the solid content of the acrylic resin after drying for 30 minutes in an oven heated to 150°C to the weight of the charged monomers by the gravimetric method, that is, the formula (Solid weight of acrylic resin / weight of charged monomer) x 100 The conversion rate was calculated from the

[0051] (Syndiotacticity in triplicate display) Using a 400MHz nuclear magnetic resonance spectrometer AVANCEIII (manufactured by Bruker), the acrylic resin was measured in a deuterated chloroform solution at 22°C with 16 cycles of accumulation. 1 H-NMR spectrum was measured. Next, the area (X) of the region from 0.60 to 0.95 ppm and the area (Y) of the region from 0.60 to 1.25 ppm when tetramethylsilane (TMS) was set to 0 ppm were measured, and then the area was calculated using the formula (X / Y)×100 The syndiotacticity of the triad was calculated by the following formula.

[0052] (glass transition temperature) The acrylic resin was heat-treated using a STA7200 thermogravimetric differential thermal analyzer (Hitachi High-Tech Science) to remove residual monomers and decomposition products of the polymerization initiator. Specifically, the acrylic resin was heated from 40 to 190°C at a rate of 10°C / min under a nitrogen flow of 200 mL / min, and then held at 190°C for 2.0 to 2.5 minutes.

[0053] The glass transition temperature of the heat-treated acrylic resin was measured using a high-sensitivity differential scanning calorimeter (DSC7000X, manufactured by Hitachi High-Tech Science). Specifically, the sample was first heated from 40°C to 160°C at a heating rate of 10°C / min under a nitrogen flow of 40 mL / min, cooled to 40°C, and then heated from 40°C to 160°C at a heating rate of 10°C / min. The midpoint glass transition temperature was then read from the DSC curve measured during the second heating cycle.

[0054] (weight average molecular weight, number average molecular weight and dispersity) The weight-average molecular weight (Mw), number-average molecular weight (Mn), and dispersity (Mw / Mn) of the acrylic resin were calculated using a high-speed GPC system HLC-8220GPC (manufactured by Tosoh Corporation). The analysis was carried out under the following conditions using a sample solution prepared by dissolving 20 mg of the acrylic resin in 10 mL of tetrahydrofuran. Detector: RI detector Solvent: tetrahydrofuran Guard column: TSKgel guard column SuperHZ-H (manufactured by Tosoh) Analytical column: TSKgel SuperHZM-H x 2 (Tosoh) Measurement temperature: 40℃ Standard material: Standard polystyrene (manufactured by Tosoh)

[0055] (contact angle with water) The contact angle of the recycled acrylic resin film with water at 25°C was measured by the drop method using a contact angle meter DMO-501 (Kyowa Interface Science).

[0056] (Preparation of acrylic resin A1) A 4L glass reactor equipped with an H-shaped impeller was charged with 150 parts by weight of deionized water, 0.20 parts by weight of tribasic calcium phosphate (as a dispersant), 0.0075 parts by weight of sodium α-olefin sulfonate, and 0.30 parts by weight of sodium chloride. Next, under a nitrogen atmosphere, 100 parts by weight of methyl methacrylate (MMA), 0.289 parts by weight of n-octyl mercaptan (as a chain transfer agent), and 0.065 parts by weight of 2,2'-azobis(isobutyrate) dimethyl V-601 (Fujifilm Wako Pure Chemical Industries, Ltd.) (as a polymerization initiator) were added to the reactor while stirring at 250 rpm. The liquid temperature in the reactor was then raised to 70°C to initiate polymerization. Two hours after the start of polymerization, 0.10 parts by weight of tribasic calcium phosphate was added to the reactor. An exothermic peak associated with the gel effect was observed 4 hours and 20 minutes after the start of polymerization. Next, heating was started 7 hours after the start of polymerization, and the liquid temperature in the reactor was raised to 95°C. The conversion rate 7 hours after the start of polymerization was 93%. Next, 2 hours after the liquid temperature in the reactor reached 95°C, the liquid temperature in the reactor was cooled to room temperature to terminate the polymerization, yielding an acrylic resin dispersion. The conversion rate at the end of polymerization was 99%. The acrylic resin dispersion was acid-washed using 0.1 times the weight of the charged monomer with 1N hydrochloric acid, and then washed with water to remove the dispersant. The washed acrylic resin dispersion was then dehydrated and dried to yield bead-shaped acrylic resin A1.

[0057] (Preparation of acrylic resin A3) Acrylic resin A3 was prepared in the same manner as in Production Example 1 of Japanese Patent No. 6827272.

[0058] (Physical properties of acrylic resin A1) Acrylic resin A1 had a triad syndiotacticity of 57%, a glass transition temperature of 120°C, a content of structural units derived from methyl methacrylate of 100% by weight, Mw of 83,000, and Mw / Mn of 1.81.

[0059] (Physical properties of acrylic resin A3) Acrylic resin A3 had a triad syndiotacticity of 75%, a glass transition temperature of 128°C, a content of structural units derived from methyl methacrylate of 100% by weight, Mw of 67,000, and Mw / Mn of 1.07.

[0060] (Preparation of acrylic resin pellets) Acrylic resin A1 or A3 was extruded at a resin temperature of 255°C using a 15 mm diameter, L / D=45 intermeshing co-rotating twin-screw extruder KZW15TWIN-45MG (manufactured by Technobel) equipped with a die at the outlet. The strand-like molten resin extruded from the die was cooled in a water tank and then pelletized in a pelletizer to obtain acrylic resin pellets.

[0061] (Acrylic resin A2) Parapet HR-S (manufactured by Kuraray) was used as acrylic resin A2 (acrylic resin pellets) with a triad syndiotacticity of 51%. Acrylic resin A2 had a glass transition temperature of 116°C, Mw of 91,000, and Mw / Mn of 1.68.

[0062] (Production of raw film) The acrylic resin pellets were dried at 90°C for 4 hours, and then extruded using a 15mm diameter, L / D=45 intermeshing co-rotating twin-screw extruder KZW15TWIN-45MG (manufactured by Technovel) equipped with a T-die at the outlet, at a resin temperature of 240°C. The sheet-like molten resin extruded from the T-die was cooled with a cooling roll to obtain a raw film having a width of 130mm and a thickness of 160µm.

[0063] (Production of easy adhesive) 20 g of Epocross WS700 (manufactured by Nippon Shokubai), a resin with an oxazoline group and a solid content of 25% by weight, was added to 100 g of Superflex 210 (manufactured by Daiichi Kogyo Seiyaku), a polyester urethane resin with a solid content of 33% by weight and a carboxyl group, and then the mixture was stirred for 3 minutes to obtain an easy-to-use adhesive.

[0064] (Production of raw film with easy-adhesion layer) After corona discharge treatment was performed on one side of the raw film, an easy-adhesion adhesive was applied using a bar coater (number 6 wire). Next, the raw film coated with the easy-adhesion adhesive was placed in a hot air dryer and dried at 80°C for about 1 minute to form an easy-adhesion layer, and a raw film with an easy-adhesion layer was obtained.

[0065] (Preparation of stretched film with easy-adhesion layer) Using a simultaneous biaxial stretching machine IMC-1905 (manufactured by Imoto Machinery Co., Ltd.), the raw film with the easy-adhesion layer was stretched at 135°C at a stretch ratio of 2 in MD and TD to obtain a stretched film with the easy-adhesion layer. The stretched film with the easy-adhesion layer had a thickness of 40 μm, and the easy-adhesion layer had a thickness of 0.35 μm.

[0066] (Examples 1 to 8, Comparative Examples 1 to 10) The stretched film with the easy-adhesion layer was shredded with a shredder to obtain small pieces of about 5 mm x 15 mm. The obtained small pieces of the stretched film with the easy-adhesion layer were acrylic resin substrates with the easy-adhesion layer attached.

[0067] In a flask equipped with a stirrer, 100 g of strips of the stretched film with the adhesive layer were immersed in 1000 ml of a 4 wt % NaOH aqueous solution under the specified conditions (see Table 1) for alkali treatment. Next, the mixture was filtered through a 200-mesh stainless steel filter, and the residue was washed with water and then dried to obtain a recycled acrylic resin.

[0068] In Comparative Examples 7, 9, and 10, no alkali treatment was performed, and purified water was used instead of the 4 wt % NaOH aqueous solution.

[0069] (Production of recycled acrylic resin pellets) Recycled acrylic resin pellets were obtained in the same manner as in (Preparation of acrylic resin pellets), except that recycled acrylic resin was used instead of acrylic resin A1 or A3.

[0070] (Production of recycled acrylic resin film) A recycled acrylic resin film was obtained in the same manner as in (Production of raw film), except that recycled acrylic resin pellets were used instead of acrylic resin pellets.

[0071] (Removal rate of easily adhesive layer) 0.1 g of recycled acrylic resin and 9 g of 4 wt% NaOH aqueous solution were placed in a test tube and heated at 90°C for 3 hours to dissolve the components of the adhesive layer remaining in the recycled acrylic resin, obtaining a solution. The resulting solution was placed in a glass cell with an optical path length of 1 cm, and the absorbance Ab1 at a wavelength of 250 nm was measured using a UV-visible spectrophotometer V-550 (manufactured by JASCO).

[0072] The absorbance Ab2 at a wavelength of 250 nm was measured in the same manner as above, except that a strip of stretched film with an easy-adhesion layer was used instead of the recycled acrylic resin.

[0073] formula (1-Ab1 / Ab2) x 100 The removal rate of the easy-adhesion layer was calculated using the following formula: It is assumed that the light absorption at a wavelength of 250 nm is mainly due to the aromatic rings of the polyester that constitutes the urethane resin contained in the easy-adhesion layer.

[0074] (comprehensive evaluation) The overall evaluation was based on the removal rate of the easy-adhesion layer and the contact angle of the recycled acrylic resin film with water. The criteria for the overall evaluation when using acrylic resin A1, A2, or A3 are as follows: A: When the removal rate of the adhesive layer is 80% or more and the contact angle with water is 65.0° or more B: When the removal rate of the easy-adhesion layer is 60% or more but less than 80% and the contact angle with water is 60° or more, or when the removal rate of the easy-adhesion layer is 80% or more and the contact angle with water is 60° or more but less than 65° C: When the removal rate of the easy-adhesion layer is less than 60%, or when the removal rate of the easy-adhesion layer is 60% or more and the contact angle with water is less than 60°

[0075] Tables 1 and 2 show the results of the removal rate of the easy-adhesion layer, the contact angle of the recycled acrylic resin film with water, and the overall evaluation.

[0076] [Table 1]

[0077] [Table 2]

[0078] From Table 1, it can be seen that in Examples 1 to 8, the hydrolysis rate of the recycled acrylic resin was low and the removal rate of the easy-adhesion layer was high, so that the recycled acrylic resin film had few defects.

[0079] In contrast, in Comparative Examples 1 to 4, the treatment parameters were 750 to 1800°C·min, resulting in a low removal rate of the easy-adhesion layer. In Comparative Example 5, the treatment parameters were 14400°C·min, resulting in a high hydrolysis rate of the recycled acrylic resin. In Comparative Example 6, the temperature of the alkaline aqueous solution was 120°C, resulting in a high hydrolysis rate of the recycled acrylic resin. In Comparative Examples 7, 9, and 10, the easy-adhesion layer was not removed because no alkaline treatment was performed. In Comparative Example 8, the syndiotacticity of acrylic resin A2 in triad notation was 51%, resulting in a high hydrolysis rate of the recycled acrylic resin even though the treatment parameters were 7200°C·min.

Claims

1. A method for producing recycled acrylic resin by removing an easy-adhesion layer from an acrylic resin substrate to which the easy-adhesion layer is attached, a step of immersing the acrylic resin substrate having the easy-adhesion layer attached thereto in an alkaline aqueous solution to perform an alkali treatment, The acrylic resin contained in the acrylic resin substrate has a syndiotacticity of 54% or more in triad expression, The temperature of the alkaline aqueous solution is 30°C or higher and 90°C or lower, A method for producing a recycled acrylic resin, wherein a treatment parameter represented by the product of the temperature of the alkaline aqueous solution and the time for immersion in the alkaline aqueous solution is 2000°C·min or more and 10000°C·min or less.

2. The method for producing recycled acrylic resin according to claim 1, wherein the acrylic resin has a syndiotacticity of 55% or more in triad expression.

3. The method for producing a recycled acrylic resin according to claim 1 or 2, wherein the acrylic resin has a content of structural units derived from methyl methacrylate of 98% by weight or more.

4. The method for producing a recycled acrylic resin according to claim 1 or 2, wherein the acrylic resin does not contain a ring structure in the main chain.

5. The method for producing a recycled acrylic resin according to claim 1 or 2, wherein the easy-adhesion layer contains a urethane resin.

6. The glass transition temperature is 120°C or higher, An acrylic resin film having a contact angle with water at 25°C of 65.0° or more and 71.5° or less.

7. The syndiotacticity of the triad is 54% or more, An acrylic resin film having a contact angle with water at 25°C of 65.0° or more and 71.5° or less.

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